Discussing originals is always of interest to me, but like most, don't have access to them to handle in person. The next best thing has been books and the internet which has added to my knowledge; especially the quality of photography available from the Auctions.
Regarding the rifle in this thread, It is actually RCA #49 after restoration. The forestock, lock and triggers are not original to this rifle as pictured in Shumway. Also the comb irregularity is wear or damage, I'm convinced it had a typical straight dickert/lancaster style comb when new.
I haven't built many rifles but my requirements for an early generic, say pre 1782(independence) would be a barrel 1 1/8 OR 1 1/16 at breech, no longer than 42in and 54 cal or larger. Buttplate min 1 7/8 wide by 5 in min heel to toe and flat or almost flat. stock with 2 1/4 to 2 5/8 drop at heel, possibly a stepped wrist, some minimal carving, wooden or simple brass patchbox lightly engraved. barrel pinned as opposed to wedges. My lock choice would be a Davis early colonial for the 1 1/8 or Chambers dale johnson for the 1 1/16
Steve